sysctl: Use header file for sysctl knob declarations on sparc.

This also takes care of a sparse warning as scons_pwroff's definition
point.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index eda6162..da51520 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -119,9 +119,7 @@
 #endif
 
 #ifdef CONFIG_SPARC
-extern char reboot_command [];
-extern int stop_a_enabled;
-extern int scons_pwroff;
+#include <asm/system.h>
 #endif
 
 #ifdef __hppa__